1. Field of the Invention
The present invention relates to a device for estimating the air pressure of a tire, and in particular, to a device for detecting a wheel speed signal frequency from the wheel speed signal of a wheel, and to a device for estimating the state of air pressure of a tire from the wheel speed frequency.
Further, the present invention relates to a device for estimating the state of air pressure of a tire, and in particular, to a device for estimating the state of air pressure of at least one of the left and right wheels from the vibration frequencies of the wheel speed signals of the left and right wheels which are connected by a mechanical element.
2. Description of the Related Art
Devices for estimating the air pressure of a tire inform the driver of abnormalities or the like in a tire of a vehicle. In addition to tire air pressure estimating devices which directly detect the air pressure of a tire, there is the tire air pressure estimating device disclosed in Japanese Patent Application Laid-Open (JP-A) No. 5-133831. The tire air pressure estimating device disclosed in JP-A-5-133831 focuses on the correlation between the air pressure of the tire and the resonance frequency of the tire. The resonance frequency of the tire is detected by a fast Fourier transform (FFT) of the vibration component of the wheel speed. When the resonance frequency is less than a predetermined value, it is judged that there is an air leak in the tire such as a puncture in the tire, and a warning is given.
Further, dynamic system diagnostic devices such as the following have been disclosed in JP-A-7-98268 and JP-A-10-187236. On the basis of the response output vector of the dynamic system, an overall disturbance vector is estimated as the sum of the external disturbance vector applied from the exterior of the dynamic system and the internal disturbance vector generated by problems within the dynamic system. The internal state amount vector of the dynamic system is estimated. By computing the cross-correlation between the estimated overall disturbance vector and the internal state amount vector, the component relating to the internal disturbance is separated from the overall disturbance vector. From this separated component relating to the internal disturbance, the corresponding portion of the dynamic system is diagnosed as a malfunctioning portion. For example, a decrease in the air pressure can be judged by using such a diagnostic device.
In this case, when the dynamic system is a wheel including a tire, the disturbance vector applied from the exterior of the wheel is a vibration input which is inputted to the wheel resonance system. If a malfunction within the tire is an internal disturbance vector generated by a decrease in air pressure, it can be judged that the air pressure of the tire has decreased.
However, the method of computing the resonance frequency in the tire air pressure estimating device disclosed in JP-A-5-133831 is complex.
Further, when the air pressure is determined by using the above-described diagnostic devices (of JP-A-7-98268 and JP-A-10-187236), there may be included a response output in which the magnitude of the response output vector becomes suddenly large, or a response output in which periodic changes are not expressed markedly. If it is determined that the air pressure has dropped on the basis of an output response which includes a response output whose magnitude has suddenly become large, the effect of the response output which has suddenly become large will be too large, and the existing, periodic response output will be hidden in the response output which became large suddenly. Accordingly, the estimated values of the physical amounts for determining the decrease in the air pressure will vary, and the accuracy of judging a decrease in the air pressure will be poor.
Further, conventionally, as described above, the relationship between the vibration frequency of the wheel speed signal and the tire air pressure is constant, and the tire air pressure is estimated by using this relationship. For example, in the tire air pressure estimating device disclosed in Japanese Patent No. 2899741, the tire air pressure is estimated on the basis of the frequency (vibration frequency) of the vibration of the wheel speed signal. Thus, the air pressure of the tire can be estimated without requiring a device for directly detecting the air pressure such as a pressure sensor.
However, in accordance with the air pressure estimating device described above, although the tire air pressure of a non-drive wheel can be estimated accurately, when the left and right wheels are connected by mechanical elements of a drive system in the case of drive wheels, the vibration component transferred from the mechanical elements and unrelated to the tire air pressure is included in the wheel speed signal. Thus, the air pressure of the tire cannot be estimated accurately.
Here, in the invention disclosed in JP-A-8-230422, by using the fact that the vibration components of the drive system transmitted from the mechanical elements are included as same-phase components in the wheel speed signals of the left and right wheels, the tire air pressure is estimated on the basis of a signal corresponding to the difference between the wheel speed signals of the left and right wheels (a signal in which the vibration component of the drive system is removed).
Namely, the above-described invention utilizes the method of least squares and the difference in the estimated disturbances of the left and right drive wheels at the time a predetermined parameter is varied, which estimated disturbances are obtained by an equation of motion of the left and right drive wheels and an equation of state of the left and right drive wheels, and estimates the aforementioned parameter, and estimates the tire air pressure which offsets the vibration component of the drive system.
However, in accordance with the above-described invention, in this device for estimating the tire air pressure of the drive wheels, the amount of computation is larger and more complex than that for non-drive wheels which are not affected by the vibration of the drive system. For example, in the above-described publication (the first embodiment thereof), the order of the matrix of the least squares method used in the tire air pressure estimating device of the non-drive wheels is 2. However, the order of the matrix of the least squares method used in the tire air pressure estimating device of the drive wheels is 4. Thus, the amount of computation which must be performed is twice as much or more. In this way, the computation processing for estimating the tire air pressure is complex.
In view of the aforementioned, a first object of the present invention is to provide a wheel speed signal frequency detecting device in which the frequency of a wheel speed signal can be determined easily, and to provide a device for estimating a tire air pressure state with high accuracy by the frequency of the wheel speed signal.
Further, in view of the aforementioned, a second object of the present invention is to provide a device for estimating a tire air pressure state by a simple computation processing.
In order to achieve the above-described first object, a first aspect of the present invention is: a device for estimating a state of tire air pressure, comprising: a wheel speed detecting device which detects a wheel speed of a wheel including a tire; a reference value setting device which sets, as a reference value, an amplitude center of the wheel speed signal outputted from the wheel speed detecting device; and a tire air pressure state estimating device which estimates a state of air pressure of the tire on the basis of a number of times the wheel speed signal crosses the reference value.
In accordance with the present invention, the wheel speed, such as the rotational speed of a wheel including a tire, is detected by a wheel speed detecting device, and the wheel speed detecting device outputs a wheel speed signal. The wheel speed detecting device may be a so-called vehicle speed sensor which detects by a sensor, such as a light sensor or a magnetic sensor or the like, a brake rotor or a gear-shaped rotor provided at the hub or the like.
The reference value setting device sets, as a reference value, the amplitude center of the vehicle speed signal outputted from the wheel speed detecting device. For example, by subtracting the value which is the amplitude center of the wheel speed signal from the wheel speed signal outputted by the wheel speed detecting device, the amplitude center of the wheel speed signal can be corrected to the zero reference. Note that the reference value is not limited to the zero reference, and may be any value which enables relative determination of the amplitude of the wheel speed signal.
Further, the wheel speed signal outputted by the wheel speed detecting device is obtained by detecting the wheel speed of the wheel including the tire. As a result, the wheel speed signal and the tire air pressure state are correlated. Further, the reference value setting device sets the amplitude center of the wheel speed signal outputted from the wheel speed detecting device as the reference value. Thus, even if the amplitude center of the wheel speed signal outputted by the wheel speed detecting device greatly varies, the number of times that the wheel speed signal crosses the reference value (number of crossings) is not affected by these fluctuations. Here, the tire air pressure state judging device of the present invention determines the number of times the wheel speed signal crosses a reference value set by the reference value setting device, and judges the tire air pressure state on the basis of the determined number of crossings. Thus, the state of the air pressure of the tire can be judged with high accuracy.
Namely, the tire air pressure state can be easily judged with high accuracy from the detected wheel speed signal, without carrying out complex computation such as fast Fourier transform computation or the like.
The reference value setting device of the present invention may set the reference value on the basis of an amount of change in the wheel speed signal in a predetermined period of time.
In this way, the present invention enables appropriate setting of the reference value by the reference value setting device setting the reference value on the basis of the amount of change in the wheel speed signal in a predetermined period of time. Accordingly, the state of the tire air pressure can be judged with high accuracy.
The first aspect of the present invention can be expressed as follows: a device for estimating air pressure of a tire of a wheel, the device comprising: (a) a wheel speed sensor operable for producing a signal, the signal having at least an amplitude center and a characteristic indicative of rotational speed of a wheel mounted on a tire; (b) a reference value setting device electronically connected to the wheel speed sensor, the reference value setting device receiving the signal and setting substantially the amplitude center of the signal as a reference value; and (c) a data processor which receives the reference value and the signal, the data processor including logic which estimates air pressure in the tire based on a number of times the signal crosses the reference value.
The reference value setting device of the present invention may include a signal converting device which converts the wheel speed signal into a signal which has an amplitude center of the signal equal to the zero reference.
In this way, the reference value setting device sets the amplitude center of the wheel speed signal as the reference, and includes the signal converting device which converts the wheel speed signal into a signal whose amplitude center is the zero reference. In this way, the number of crossings of the wheel speed signal and the reference value can be computed by counting the number of times the sign changes, and computation processing is made easy. Accordingly, the state of the air pressure of the tire can be judged easily and with high accuracy.
The reference value setting device of the present invention may include an amplifying device which amplifies the vehicle speed signal.
In this way, in the present invention, the reference value setting device sets the amplitude center of the wheel speed signal as a reference, and includes the amplifying device which amplifies the wheel speed signal. Truncation errors can thereby be suppressed when the number of crossings of the wheel speed signal and the reference value is computed, and the accuracy of computation can be improved.
The present invention may include a passing device which allows signals only of a predetermined frequency band of the wheel speed signal to pass; and a correcting device for correcting the predetermined frequency band of the passing device on the basis of the frequency detected by the frequency detecting device which detects the frequency of the wheel speed signal.
In this way, in the present invention, the passing device eliminates frequencies other than that of the predetermined frequency band. Namely, components (noise) which are included in the wheel speed signal and are not needed to estimate the tire air pressure state can be eliminated. Further, the correcting device corrects the predetermined frequency band which passes through the passing device on the basis of the frequency of the wheel speed signal detected by the frequency detecting device. Thus, noise included in the wheel speed signal can be removed effectively.
Here, the correcting device can effect correction such that the center of the predetermined frequency band of the passing device equals the frequency detected by the frequency detecting device.
In the present invention, the correcting device makes the center of the predetermined frequency band of the passing device to equal the frequency detected by the frequency detecting device. In this way, the frequency components other than the frequency component of the frequency band needed for detection at the frequency detecting device are eliminated from the wheel speed signal passing through the passing device. Accordingly, the air pressure state can be determined with high accuracy.
Further, the frequency detecting device may detect the frequency of the wheel speed signal on the basis of the number of crossings.
As described above, because the wheel speed of the wheel including the tire is detected, the wheel speed signal outputted from the wheel speed detecting device is correlated with the resonance frequency of the tire. In other words, the frequency of the detected wheel speed signal also is correlated with the tire resonance frequency. Accordingly, on the basis of the frequency of the detected wheel speed signal, the passing device can be set to a predetermined frequency band including the tire resonance frequency.
Further, the frequency detecting device may detect the frequency of the wheel speed signal by the equation f=C/(2TN), wherein the measured time of one wheel speed signal is T, the number of measured samples of wheel speed signals is N, and the number of crossings is C.
The present invention may include a computing device which computes a determination value for selecting the wheel speed signals; and a selecting device for selecting the needed wheel speed signals on the basis of the determination value.
Namely, the determination value for selecting the wheel speed signal is computed by the computing device. On the basis of the determination value, the needed wheel speed signal is selected by the selecting device. Accordingly, the selecting device can select the wheel speed signals having good S/N ratios when the computing device is configured to compute a determination value for selecting signals which includes little noise. Namely, since the tire air pressure state can be judged by the selected wheel speed signal, the tire air pressure state can be judged with high accuracy.
The computing device may compute, as the determination value, a comparison value for comparing the number of crossings of the wheel speed signal and the reference value with the number of crossings of the reference value and the converted signal which has been converted so as to be changed in accordance with changes in the wheel speed signal.
Here, for example, when the number of crossings is determined as described above from a wheel speed signal including noise which becomes large suddenly, the determination value includes many components other than periodic components. Thus, if the wheel speed signal includes noise which becomes large suddenly, the number of times the wheel speed signal crosses the reference value, and the number of times a signal, which has been converted so as to be changed in accordance with changes in the wheel speed signal, crosses the reference value, are different. Accordingly, the wheel speed signals are selected on the basis of a comparison value between the number of times the wheel speed signal crosses the reference value and the number of times a signal, which is converted so as to vary in accordance with changes in the wheel speed signal, crosses the reference value. Thus, wheel speed signals having good S/N ratios can be selected.
Here, the conversion which is carried out such that the wheel signal is varied in accordance with variations in the wheel speed signal may be an nth order difference or an nth order differentiation (wherein n is a natural number).
In this way, noise whose magnitude is extremely large and which is included in the wheel speed signal can be eliminated. Further, by making n equal to about 2, the load of computation for conversion is low, and the noise can be effectively removed.
The state of the air pressure to be estimated may be a state in which the tire air pressure has decreased.
Namely, if the number of times the wheel speed signal and the reference value cross at a time of normal air pressure or a frequency of the tire speed signal at the time of normal air pressure is measured in advance, it is possible to determine whether the tire air pressure state has decreased from the number of times the wheel speed signal crosses the reference value or from the frequency of the wheel speed signal.
The frequency of the wheel speed signal can be detected from the number of times the wheel speed signal crosses the reference value. Thus, the following second aspect of the present invention is proposed. This second aspect of the present invention includes a device for detecting a wheel speed signal frequency, comprising: a wheel speed detecting device which detects a wheel speed of a wheel including a tire; a reference value setting device which sets, as a reference value, an amplitude center of the wheel speed signal outputted from the wheel speed detecting device; and a frequency detecting device which detects a frequency of the wheel speed signal on the basis of a number of times that the wheel speed signal crosses the reference value.
In the second aspect of the present invention, the frequency of the wheel speed signal is detected from the number of times the wheel speed signal crosses the reference value. Thus, the frequency of the wheel speed signal can be detected without carrying out complex computation such as fast Fourier transform or the like.
Next, a third aspect of the present invention will be explained. The third aspect is an aspect intended to achieve the above-described second object, and is a device for estimating a tire air pressure state, comprising: a wheel speed signal detecting device which detects a wheel speed signal of each of left and right wheels including a tire; a first vibration frequency estimating mans which estimates a vibration frequency of each of the wheel speed signals of the left and right wheels; a computing device which computes a difference between the wheel speed signals of the left and right wheels; a second vibration frequency estimating device which estimates a vibration frequency of the difference between the wheel speed signals of the left and right wheels; and an estimating device which estimates a tire air pressure state of at least one of the left and right wheels, on the basis of each of the vibration frequencies of the wheel speed signals of the left and right wheels estimated by the first vibration frequency estimating device, and the vibration frequency of the difference between the wheel speed signals of the left and right wheels estimated by the second vibration frequency estimating device.
The wheel speed signal detecting device detects the wheel speed signals of the left and right wheels. The left and right wheels may be connected by mechanical elements.
The first vibration frequency estimating device estimates the vibration frequency of each of the wheel speed signals of the left and right wheels.
The computation device computes a difference between the wheel speed signals of the left and right wheels. The second vibration frequency estimating device estimates the vibration frequency of the difference between the wheel speed signals of the left and right wheels computed by the computing device.
A vibration component extracting device may further be provided which extracts the vibration components of the wheel speed signals of the left and right wheels detected by the wheel speed signal detecting device. The first vibration frequency estimating device and the, computing device may use, as the wheel speed signals for the left and right wheels, vibration components of the wheel speed signals of the left and right wheels extracted by the vibration component extracting device.
The estimating device estimates the tire air pressure state of at least one of the left and right wheels on the basis of the vibration frequencies of the wheel speed signals of the left and right wheels estimated by the first vibration frequency estimating device, and on the basis of the vibration frequency of the difference between the wheel speed signals of the left and right wheels estimated by the second vibration frequency estimating device. The estimating device may estimate the tire air pressure itself of at least one of the left and right wheels, or may estimate a decrease in the tire air pressure of at least one of the left and right wheels.
Here, generally, the difference between the wheel speed signals of the left and right wheels computed by the computing device corresponds to the difference between the wheel speed signals from which have been removed the effects of the vibrations of the same phases of the left and right wheels. The vibration frequency of the difference between the wheel speed signals of the left and right wheels estimated by the second vibration frequency estimating device corresponds to an intermediate frequency of the frequencies of the left and right wheel speed signals, from which has been removed the effect of vibrations of the same phases caused by the left and right wheels being connected by mechanical elements.
Accordingly, on the basis of the vibration frequency of the difference between the wheel speed signals of the left and right wheels estimated by the second vibration frequency estimating device which is an intermediate frequency of the frequencies of the left and right wheel speed signals, from which the effect of the vibrations of the same phase has been removed, the estimating device of the present invention corrects the vibration frequencies of the wheel speed signals of the left and right wheels estimated by the first vibration frequency estimating device, which includes the effects of the vibrations of the same phases of the left and right wheels.
Namely, the estimation device corrects each of the vibration frequencies of the wheel speed signals of the left and right wheels estimated by the first vibration frequency estimating device, by an average value of differences between, on the one hand, each of the vibration frequencies of wheel speed signals of left and right wheels estimated by the first vibration frequency estimating device, and on the other hand, a vibration frequency of a difference between wheel speed signals of left and right wheels estimated by the second vibration frequency estimating device. On the basis of the corrected vibration frequencies of the wheel speed signals of the left and right wheels, the tire air pressure state of at least one of the left and right wheels is estimated. Accordingly, the vibration frequencies of the wheel speed signals of the left and right wheels can be estimated with high accuracy, and thus, the tire air pressure state of at least one of the left and right wheels can be estimated with high accuracy.
In the present invention as described above, the tire air pressure state of at least one of the left and right wheels is estimated on the basis of the vibration frequencies of wheel speed signals of the left and right wheels and on the basis of a vibration frequency of a difference between the wheel speed signals of the left and right wheels. Thus, the tire air pressure state can be estimated without carrying out complex computation processing.
Here, the left and right wheels may be both drive wheels or both non-drive wheels of a vehicle. In the present invention, the air pressure states of the drive wheel and the non-drive wheel can both be estimated by using the same method. Namely, a common computation processing can be used for both. In this way, because there is no need to provide separate devices for a drive wheel and a non-drive wheel, the structure of the device can be simplified.